2x SLI GTX280's is still more than acceptable at 1080p with even the latest games, so absolutely no need to change them (unless you have the money to burn).

I would recommend against that motherboard for two reasons:

1. The secondary PCI-Ex 16x slot is 4x speed when used as well as 1st slot. 8x speed is very little noticable difference, but 4x will be.
2. You have no SLI support for your existing GPUs, which as stated are more than good enough.